When a fossil-fueled power station is being started or started up, the boiler of the power station is first of all raised to a minimum load (generally 30 to 40%). The fresh steam which is produced during this starting-up phase in this case normally initially bypasses the steam turbine in the (so-called) bypass mode. In the case of installations having intermediate superheating, the fresh steam is in this case passed via a high-pressure bypass station, is reduced to a lower temperature level, and is then passed to the cold branch of the intermediate superheating. The steam leaving the hot branch of the intermediate superheating is passed via a medium-pressure bypass station and, after being cooled by means of injected water, is passed to the condenser. A high pressure level in the intermediate superheating (normally about 20-30 bar) in this case ensures effective cooling of the intermediate superheating tubes, to which flue gas is applied.
When a high-pressure turbine in the steam power station is accelerated to the rated rotation speed from this bypass mode as described above, then the high pressure in the cold branch of the intermediate superheating leads, at the outlet of the high-pressure turbine, to temperatures which are considerably higher than during rated load operation, particularly in the case of hot starting or warm starting. The reason for this is the small temperature decrease and small amount of surging in the high-pressure turbine when the mass flows are low. This no-load mass flow cannot be increased, because of the rotation-speed regulation, since the turbine-generator run cannot yet emit any power to the network. During this phase, the turbine produces only the power loss in the bearings and generator which, depending on the installation size, is normally in the range from 2 to 5 MW. This power cannot be increased until after synchronization to the network.
The high temperatures which therefore occur before synchronization make it necessary to design the waste-steam area of the high-pressure turbine and the line of the cold branch of the intermediate superheating such that they withstand the increased temperatures, in particular also the temperatures, which change to a major extent during start-up and shut-down. At the moment, this is possible by the use of relatively cost-effective materials in the design of the turbine and the line of the cold branch of the intermediate superheating. However, during hot starting in future installations, in order to increase the fresh-steam temperatures of about 565° C. that are normally used nowadays with an associated high-pressure waste-steam temperature of at most approximately 500° C. to a maximum of about 700° C. with associated waste-steam temperatures of about 580° C. to 600° C. at times, it is necessary to also use considerably more expensive materials, in particular 10%-Cr-steel, in the high-pressure waste-steam area and in the cold branch of the intermediate superheating.
Other known solutions are following the aim of suitable cooling. For example, in the past, so-called start-up lines had been used, which connect the high-pressure waste-steam area directly to the condenser, for start-up. In this case, the expansion line is lengthened and surging in the high-pressure turbine is prevented by reducing the high-pressure waste-steam pressure during start-up and no-load running. However, an additional, relatively large line and water injection are required for this purpose. It is also known for other start-up concepts to be pursued. For example, it is known for flue gas to bypass intermediate superheated tubes via boiler valves. These tubes therefore need not be cooled, and the steam turbine can be started up with very low pressures in the cold branch of the intermediate superheating. In another known start-up concept, the high-pressure turbine first of all runs in an evacuated form, and is connected to the network only after synchronization.
Considered overall, the cooling solutions and start-up concepts described above as well as the inclusion of heat-resistant materials are highly complex and costly, thus resulting in a need for better solutions in order to reduce the high temperatures which occur before network synchronization.